The present invention relates to a device for terminating logic signals to peripheral devices from a computer or controller and more particularly for terminating logic signals to a plurality of peripheral devices from a computer or controller wherein there is minimal line reflection.
Peripheral devices connected to a computer may include paper tape, keyboards, disk drives, display devices, and output printers. The interface between the computer and the peripheral device is generally characterized by a multi-way cable carrying high speed logic signals. The most critical signals are transmitted via dedicated differential lines with an appropriate termination impedance. The less critical control signals are often transmitted on a transmission line which may be multiplexed between several peripheral devices connected by a connecting cable. The selection of an individual peripheral device is accomplished by means of a coded signal transmitted on one or more control lines.
It is necessary to provide a terminating impedance on each such line which matches the characteristic impedance of the connecting cable used, in order to avoid line reflection. This termination is generally provided by means of a 330 ohm resistor connected to a logic ground and a 220 ohm resistor connected to the control or transmission line. That provides an effective impedance of 132 ohms, which closely matches the characteristic impedance of the commonly-used twisted-pair or flat-ribbon connecting cable and provides a dc bias appropriate to the matching characteristics of the transmitting and receiving devices.
In a system where only one peripheral device is connected to the transmission line, that peripheral device must also have such a termination network in order to avoid line reflection problems. Where a multiplicity of peripheral devices are connected to the same transmission line, then only one of the peripheral devices need have a termination network. The effect of paralleling several termination networks would reduce the effective terminating impedance to an incorrect value and would place an excessive load on the logic drive element.
In most present disk drive designs, a termination having a socket connected to each line to be terminated is used. Into the socket may be plugged the common point of the 220 ohm, 330 ohm resistor combination whose other ends are connected through the socket to ground and to an appropriate power supply voltage, respectively. To connect the termination network to a particular peripheral device, an appropriate resistor array is plugged into such socket. If no termination network is required, the resistor array is omitted. In practice, this is provided by using a single in-line or dual in-line resistor array and a matching socket.
In many peripheral device designs however, for example, in some disk drive designs, space or other considerations demand that access to the printed circuit board containing the termination package be restricted. The circuit board, for example, may be mounted with the components facing away from the outside toward some other part of the disk drive structure. In such circumstances, it is thus necessary to remove or to partially dismantle the disk drive in order to gain access to the termination network. Some disk drive designs have attempted to resolve this problem by utilizing an angled socket so that, using appropriate tools, such as needle nose pliers, a termination package may be removed or inserted from the exposed edge of the board.